1. Field of the Invention
The present invention relates generally to electrical circuitry and, more particularly, to methods and apparatus for forming solder balls.
2. Description of the Related Art
In the fabrication of electrical circuits, it is often desirable to couple a circuit component to a substrate. For example, an integrated circuit chip may be coupled to a ceramic substrate or to a printed circuit board. A method commonly used for the surface mounting of a circuit component involves the placement of solder balls between the circuit component and the substrate. The assembly is then heated to melt the solder balls to couple the circuit component to the substrate.
To facilitate this type of surface mounting technique, the height of the solder balls should be substantially uniform. If the height of the solder balls is not uniform, the smaller balls may fail to wet, i.e., adhere, to either the circuit component or the substrate, thus causing the failed formation of the desired electrical connection. Due to the criticality of the size of the solder balls, various techniques have been used in an effort to obtain uniform solder balls.
Most commonly used methods of forming solder balls employ a stencil and operate quite similarly to a screen printing process. The stencil includes a pattern of holes which are formed in it, and each of these holes define a volume that is substantially identical to each other hole. The hole pattern typically corresponds with a pattern of electrical contacts to be formed between the circuit component and the substrate.
In this process, solder is applied in the form of solder particles, such as tin/lead particles, which are held together in a flux paste. The stencil is placed over and slightly spaced above the substrate, so that the holes in the stencil are positioned over the contact pads on the substrate. Once in position, the solder paste is placed on the stencil. As a squeegee moves along the stencil and over the holes, it deposits the solder paste into the holes and presses the stencil into contact with the substrate below. When the stencil flexes back into position spaced above the substrate, it leaves plugs of solder paste on the contact pads of the substrate. After the squeegee has completed its stroke along the stencil, the stencil is removed. The substrate is then heated so that the solder reflows to form solder balls which adhere to the contact pads.
The general technique described above suffers from various drawbacks. First, the holes in a stencil should be of uniform size and volume, otherwise the solder balls will not be of uniform size. Second, if a flexible squeegee, such as a rubber squeegee, is used, the squeegee tends to “scoop out” a small portion of the solder paste near the top of each hole in the stencil because the flexible material tends to flex downwardly into each hole. The “scoop out” alters the desired volume of solder paste, thus affecting the ultimate size of the solder balls created. Third, some of the solder paste tends to stick to the walls of the holes of the stencil. Thus, when the stencil flexes back into position to leave a plug of solder paste on the substrate, the volume of solder paste left on the substrate tends to be less than the volume of the hole in the stencil. Indeed, the variations between the plugs of solder left on the substrate may be quite significant, thus precluding the formation of a number of balls having uniform size.
In an effort to address this first problem, various types of methods have been used to create uniform holes in stencils. For instance, the holes in some stencils are formed by performing a chemical etch from both sides of the stencil. While the holes formed in this manner tend to be quite uniform, chemically etched stencils unfortunately tend to have holes bordered by small cusps. These cusps tend to retain the solder paste, which eventually plugs the holes in the stencil during repeated use. Alternatively, lasers may be used to cut holes having straight walls in a stencil. Such holes tend to be very uniform in size and they tend to retain less solder paste than holes cut with the chemical etching process. Nonetheless, laser cut holes will retain some solder paste and eventually become plugged as well.
In an effort to address the second problem, relatively inflexible squeegees, such as metal squeegees, have been utilized. Although relatively inflexible squeegees solve the problem of “scoop out,” such inflexible squeegees tend to stretch the stencil, thus slightly altering the volume and position of each hole and, eventually, rendering the stencil useless.
In an effort to address the third problem, E-fab stencils have been developed. E-fab stencils have trapezoidally or frustroconically shaped holes that widen toward the bottom of the stencil. Due to the shape of the holes, E-fab stencils tend to plug less frequently than the chemical etch or laser cut stencils mentioned above. However, due to the manner in which all stencils are used, some solder paste still sticks to the walls of the openings as the stencil is removed. Thus, regardless of the type of stencil used, the solder paste builds up on the stencil walls so that the plugs of solder paste remaining on the substrate after stencil removal become smaller or nonexistent with repeated use of the stencil.
The present invention may address one or more of the problems discussed above.